The present invention relates to a method and apparatus for varying the width of clearances between the rolls of a calender. More particularly, the invention relates to a method and apparatus for rapidly increasing the width of clearances between neighboring rolls of a calender wherein the rolls form a normally vertical row of superimposed rolls and at least some of the rolls are movable sideways. Still more particularly, the invention relates to improvements in a method and apparatus for manipulating the rolls of a calender wherein at least one of the outermost rolls can be moved toward and away from the neighboring roll, preferably by pressurized fluid.
It is already known to separate the rolls of a calender which is used for the treatment of webs consisting of paper, textile or synthetic plastic material. Separation is desirable in order to prevent the neighboring rolls from damaging each other in the absence of a web therebetween. The width of clearances which develop upon separation of neighboring rolls should be sufficient to prevent direct contact between such rolls. The interval which is needed to increase the width of clearances should be as short as possible in order to reduce the period of potential damage to the rolls to a minimum. As a rule, the interval which elapses after a web breaks and before the rolls are properly separated from each other should be less than one second. Separation by moving the lower rolls away from the rolls thereabove is preferred at this time because it takes up less time than the lifting of each higher roll above and away from the roll therebelow.
In certain presently known calenders, the bearings for the lowermost roll rest on the pistons of hydraulic cylinders. A detector monitors the running web and transmits a signal which initiates the movement of lowermost roll downwardly and away from the next-to-the-lowermost roll. Such movement is achieved by opening paths for the escape of hydraulic fluid from the chambers below the pistons. For example, the signal from the detector can initiate opening of normally closed solenoid-operated valves in conduits which connect the aforementioned chambers with a tank for hydraulic fluid. The downward strokes of pistons (upon opening of the paths) equal or closely approximate the combined width of clearances between neighboring rolls when the rolls are separated to an extent which is needed or deemed necessary to avoid damage to the rolls in the absence of a web therebetween. The lower end position of the lowermost roll is determined by one or more stops for the pistons, for example, by stops which form part of the cylinders and against which the pistons abut upon completion of their movement in a direction to allow the lowermost roll to descend by gravity and to the extent which is needed for sufficient separation of neighboring rolls from each other. If the lowermost roll is movable between the lower end positions in one of which the rolls are separated sufficiently to avoid damage thereto in the absence of a web therebetween and in the other of which the width of clearances suffices to allow for removal of a roll from the calender frame or stand, the calender comprises one or more removable stops which intercept the pistons for the lowermost roll in the one lower end position and which, when removed, allow the pistons for the lowermost roll to descend to the other lower end position. Alternatively, the calender can be equipped with a limit switch or the like which closes the valves in the paths for the escape of fluid from the aforediscussed chambers when the pistons for the bearings of the lowermost roll reach the one lower end position.
The cylinders for the pistons which support the bearings for the lowermost roll can be designed to allow for raising or lowering of such bearings as well as to furnish the pressure which, in normal operation, is needed to bias the rolls against the web portions which extend through the nips of neighboring rolls. Alternatively, the cylinders for pistons which support the bearings for the lowermost roll can serve the aforediscussed purpose to vary the width of clearances between neighboring rolls in the case of an emergency (normally a break in the running web) as well as to bias the bearings against a stop when the calender is in use. The force with which the rolls bear against the running web is then supplied by another device.
It has been found that the length of intervals which elapse in presently known calenders during separation of the rolls is excessive, i.e., that such intervals are sufficiently long to enable the neighboring rolls to damage each other in the event of breakage of the running web. In such conventional calenders, the length of intervals which are needed for separation of rolls depends on a number of factors, such as the design, number and length of rolls. Furthermore, such intervals are longer than desirable because the movement of the lowermost roll away from the roll thereabove must be braked practically immediately after the lowermost roll begins to move downwardly. The braking action is furnished by the aforementioned valves which control the flow of hydraulic fluid from the chambers of cylinders during downward movement of pistons to thereby lower the bearings for the lowermost roll. The speed of downward movement of the lowermost roll, especially during the last stage of such movement, cannot exceed a certain value because a sudden impact of bearings for the lowermost roll against stationary stops while the bearings descend at a relatively high speed could result in generation of substantial forces (due to inertia of the large mass of material which moves downwardly) which could cause damage to or destroy the lowermost roll and/or its bearings. Moreover, the median portion of the lowermost roll would be likely to flex to such an extent as to strike against the median portion of the roll immediately thereabove. Such rebounding or backlash of the median portion of the lowermost roll could cause more damage than that which develops if the rolls are not separated from each other when the web breaks.